This invention relates to a closure for a container of tennis balls.
The modern tennis ball, which usually comes packaged three (sometimes four) balls in a metal or plastic cylindrical can, is rather durable. It is common for the tennis balls from a single can to remain in good playing condition for four or five matches, amounting to perhaps six to eight hours of play. However, once the condition of a tennis ball has deteriorated, the ball is no longer either enjoyable or easy to play with. Playing with such balls can also be very bad for one's game: the inconsistent bounces and lack of control of spent tennis balls can lead to poor tennis strokes and form; these same problems can also risk or aggravate tennis injuries, such as tennis elbow. Finally, it can be embarrassing, as well as annoying, to arrive at courtside for a match only to find that the tennis balls one has brought are in poor condition for play.
Some tennis players try to determine the playability of their tennis balls by inspection: they look at the felt covering of the ball to see if it is worn, they bounce the ball from various heights to determine rebound, and they squeeze it to check deflection. For a variety of reasons, however, this sort of examination can be misleading, as well as difficult. Often a tennis ball will still appear fluffy, rather than worn, after it has been bashed about for some time. This is because the felt covering (usually a woven blend of wool and nylon) can puff up; but too much of this fluff is undesirable because it increases the ball's wind resistance, thus slowing it down. Determining whether a ball has enough rebound involves dropping it, several times, from a fixed height onto the court surface and measuring the exact height of its bounce. To check whether a ball has enough deflection, one must squeeze it with a specificed force and note how much it deflects. It is obvious that these tests, even if they can be done accurately, are very time-consuming--especially since one often wants to compare the balls from several open cans and to choose those in the best condition.
In tennis tournaments all of these problems are avoided, and good-playing tennis balls are assured, by the requirement that new cans of balls be opened after a fixed number of games have been played (usually nine). For the average, noncompetitive player this custom of replacing the tennis balls after only a few games is neither economical (because of the expense of tennis balls) nor necessary (because of their durability). With the average player a tennis ball's useful life can be measured in a few matches or several sets or hours of play, the exact number being fairly constant for a given individual, though varying among players depending upon their level of skill and the type of court surface and brand of ball they use. In fact, replacement of a can of tennis balls after such a fixed amount of use is just what the typical tennis player tries to do. For the average player, however, there is often a period of some days, or even weeks, between successive matches; the average player also often has more than one opened can of tennis balls in his possession. For these reasons it is often very difficult to remember exactly how much use a particular can of tennis balls has had.